Touchpad stylus having isolated low-mass contact element

ABSTRACT

A touchpad stylus includes a housing, a stylus member axially moveable in the housing, and an energy-absorbing cushion member interposed between a rear extremity of the stylus member and an internal cavity of the housing, the cushion member lightly forwardly biasing the stylus member relative to the housing. The stylus member has a smoothly spherical tip extremity, and the device avoids impact damage to fragile outer membranes of existing touchpad devices because the full mass of the housing is isolated from impact with the touchpad, the stylus member itself having very significantly less mass than an aggregate mass of the complete assembly.

RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.10/186,323, filed Jun. 27, 2002, now U.S. Pat. No. ______, entitled“Touchpad Stylus Having Isolated Low-Mass Contact Element,” the contentsof which are all incorporated by reference herein in their entirety.

BACKGROUND

The present invention relates to electronic pointing devices and writingimplements, and more particularly to styli for use with graphic tabletssuch as touchpads.

In the art of touchpads, it is conventional to provide a thin, flexiblemembrane as an outer member to be contacted by a stylus that is held bya user of the touchpad. A particularly vexing problem with thisarrangement is that the flexible membrane, in order to permithigh-resolution pointing and/or writing, is typically very thin anddelicate, being particularly subject to damage in the event that theuser subjects the touchpad to significant impacting contact by thestylus, which necessarily also has a fine point for operating thetouchpad at high resolution.

It is known to use a conventional ball point pen cartridge as a styluspoint, when it is desired to produce an inked record (such as on a paperstrip temporarily covering the touchpad). When the cartridge is in goodcondition the rolling action of the ball avoids sliding contact wear ofthe membrane and/or the cartridge. However, when ink is not desired, anempty cartridge generally does not provide this advantage because theball normally fails to roll. More importantly, the touchpads in use arestill subject to excessive damage even when the stylus incorporates aball pen cartridge.

In my previously-filed patent application (U.S. patent application Ser.No. 10/186,323, now U.S. Pat. No. ______), I disclosed a touchpad styluswhich provides smoothly increasing axial displacement of the stylusmember of up to approximately 0.05 inches in response to axial loadsranging from approximately 0 to up to approximately 200 grams. Thistouchpad stylus was based on the general belief that the “normal”pressure applied by a signator is typically less than about 200 grams.Recent test work, however, has revealed that a number of individualsoccasionally press down against a touchpad with increased force, such asbetween 200 grams and 500 grams and occasionally between 500 grams andabout 1000 grams.

Accordingly, there is a need for a touchpad stylus that does not suggestouter membranes of touchpads to excessive damage even when forces ofbetween about 200 grams and about 1000 grams are applied.

SUMMARY

The present invention meets this need by providing a stylus device thateliminates or greatly reduces the damage to the thin outer membranes oftouch pads in normal use. It has been discovered that the bulk of damageto touchpads in use occurs at the initial impact of the stylus againstthe membrane. The stylus device of the present invention avoids thisproblem by limiting forces of impact when the stylus is brought intocontact with the touchpad or similar device.

In one aspect of the invention, the stylus device includes an elongatestylus housing; a stylus body having a tip portion for contacting thetouchpad device, the body being supported by the housing for axialmovement along the longitudinal axis; a stop for limiting forwardmovement of the stylus body relative to the housing; and biasing meansfor forwardly biasing the body with a force being limited to apredetermined maximum axial bias over a predetermined minimum traveldistance of rearward movement relative to a rest position of the stylusbody, the minimum travel distance being not less than 0.05 inch, themaximum axial bias being not more than approximately 1000 gm. Preferablythe stylus body has a mass that is less than 10 percent of an aggregatemass of the stylus device for significantly limiting impact loading ofthe touchpad device upon initial contact by the stylus body. Preferablythe mass of the stylus body is not greater than 0.5 gm.; more preferablythe mass of the stylus body is not greater than only approximately 0.2gm. Notwithstanding the above, the stylus body preferably has a massthat is less than only approximately 5 percent of an aggregate mass ofthe stylus device. As used herein, the term “stylus body” is inclusiveof the above-recited tip portion and any rigid elements rigidlyassembled therewith.

The biasing means can include a resilient cushion member interposedbetween a forwardly facing cavity surface of the housing and a rearextremity of the stylus body.

The rear extremity the stylus member can form an engagement surfacehaving a progressively increasing cross-sectional engagement area overan axial distance not less than the minimum travel distance, the cushionmember having a contact area facing the stylus member, the contact areabeing greater than a maximum of the engagement area. The engagementsurface can include a face portion and a tapered portion, the faceportion being generally planar and oriented approximately perpendicularto a longitudinal axis of the housing. The tapered portion of theengagement surface can be approximately conical. The face portion of theengagement surface can have an area that is approximately 25 percent ofthe maximum of the engagement area, and the contact area of the cushionmember is preferably at least approximately 4 times the maximum of theengagement area for enhanced cushioning by the cushion member.Preferably the cushion member also has an axial depth that is not lessthan double the minimum travel distance. Further, the maximum of theengagement area is preferably not more than approximately 0.01 squareinch.

Preferably the minimum travel distance of the stylus body relative tothe housing is at least approximately 0.05 inch and the maximum axialbias is not more than approximately 1000 gm.

The stop can include a resilient stop member interposed between ahousing stop surface of the housing and a stylus stop surface of thestylus body. The stylus body can have a cylindrical stem portion, withthe stylus stop surface forming a generally annular enlargement of thestylus body, and the housing stop surface also being generally annularand facing the stylus stop surface, the stylus body protruding throughthe stop member. The stop member can include a polymeric O-ring.

The stylus body can include a polymeric material that preferably has africtional coefficient not greater than 0.2, as measured dry and withthe material contacting steel. More preferably, the coefficient offriction is not more than approximately 0.1. The polymeric material ofthe stylus body can be an acetal resin. Preferably the tip portion ofthe stylus body forms a contact surface having a radius of curvaturethat is not less than 0.5 mm for limiting localized stress at locationsof contact on the touchpad device. More preferably, the radius ofcurvature of the contact surface is at least approximately 1.0 mm.

The stylus device can be provided combination with the touchpad device,the touchpad device having an active surface and being responsive toforce locally applied against the active surface, and further comprisinga flexible sheet material removably covering the active surface toprovide a replaceable writing surface to be contacted by the tip portionof the stylus. Preferably the flexible sheet material has a permanentlyapplied adhesive coating for affixing the sheet material to the touchpaddevice.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a perspective view of touchpad stylus according to the presentinvention, the stylus being positioned against a touchpad for usetherewith;

FIG. 2 is a lateral sectional view of the stylus of FIG. 1;

FIG. 3 is a detail sectional view within region 3 of FIG. 2;

FIG. 4 is a detail view of a stylus head portion appearing in FIG. 3;

FIG. 5 is a sectional view as in FIG. 3, showing the stylus in a loadedcondition;

FIG. 6 is a graph of a progressively increasing biasing force of thestylus of FIG. 1 against rearward movement of a stylus member of thestylus relative to a housing thereof; and

FIG. 7 is a detail view as in FIG. 3, showing an alternativeconfiguration of the stylus head portion.

DESCRIPTION

The present invention is directed to a stylus for use with touchpads andsimilar devices wherein a stylus point or tip is positioned and/or movedon a surface in electronic pointing operations or the production ofgraphical data. With reference to FIGS. 1-6 of the drawings, a touchpadstylus 10 includes a stylus housing 12 and a stylus member 14 having atip extremity 16 that is adapted for contact with an outer membrane 18of a touchpad 20, the membrane 18 covering a pad member 19. It will beunderstood that details of the touchpad 20 are outside the scope of thepresent invention, other than that typical touchpads includecounterparts of the membrane 18, being a somewhat delicate member thatis subject to damage when used with many styli of the prior art. Asfurther shown in FIG. 1 and described below, the touchpad 20 can beprovided with an optional thin replaceable sheet member 21 covering theouter membrane 18.

The stylus housing 12 has a rear or body portion 22 and a threadinglyconnected front or nose portion 24 which together form a convenientlyhand-held device that is generally symmetrical about a longitudinal axis25, the nose portion having a passage 26 formed therethrough forslidingly receiving a cylindrical stem or shank portion 28 of the stylusmember 14 in concentric relation to the axis 25, the tip extremity 16projecting forwardly of the nose portion 24. The stylus member 14 alsohas an enlarged head portion 30 that forms a forwardly facing stylusstop surface 32 extending outwardly from a rear extremity of the shankportion 28, the nose portion 24 of the housing 12 also having arearwardly facing housing stop surface 34. A resilient O-ring 36 issupported on the shank portion 28 of the stylus member, being interposedbetween the stylus and housing stop surfaces 32 and 34 for cushionedlimitation of forward travel of the stylus member 14 relative to thehousing 12. The tip extremity 16 is formed having a smooth convexprofile, a preferred profile being of spherical radius R. The radius Ris sufficiently large to appropriately limit localized compressivestress and shear loading of the outer membrane 18 and/or the optionalsheet member 21, while permitting a desired degree of visual access toportions of the touchpad 20 to which the tip extremity 16 is beingapplied. For this purpose, the radius R can range from relatively small(0.2 mm for example) to relatively large (0.75 mm for example); however,as a result of experimental testing it has been found that a range offrom 0.25 mm to 0.5 mm is preferred, being equivalent to a spherical tipdiameter of from 0.5 mm to 1.0 mm.

Preferably the stylus member 14 is formed of a material havingrelatively low density yet being sufficiently hard to retain its shapein normal use, the material also having a very low coefficient offriction with the outer membrane 18 of the touchpad 20 and/or the sheetmember 21 optionally applied thereto for minimal wear of thosematerials. Further, the stylus member (or at least that portionincluding the tip extremity 16) is also formed of a material having highwear resistance for preserving a desired shape of the tip extremity 16.Commercially available materials particularly suited for use in thestylus member 14 include formulations of DELRIN® acetal resin that areavailable from a variety of sources. Of particular interest are Delrin500, 550, and 507, having a coefficient of friction of betweenapproximately 0.1 and 0.3, both static & dynamic as measured dry, onsteel. Similarly, Delrin AF has a coefficient of friction betweenapproximately 0.05 and 0.15, and Delrin 570X has a coefficient offriction of approximately 0.2. Another formulation *?* of Delrin acetalresin has a coefficient of friction of approximately 0.25 and a wearfactor of 216, which compares favorably with an alternative material,nylon, having a friction coefficient of 0.35 and a wear factor of 72.Accordingly, it is preferred that the material of the stylus member beor include a polymer having a coefficient of friction not greater than0.2, which is believed to be easily obtainable by selection fromavailable stock. Further, a most preferred material for the stylusmember 14 is Delrin AF, selected for having a friction coefficient ofnot greater than approximately 0.1.

The housing body portion 22 has a forwardly facing cushion cavity 38formed therein for supporting a cushion member 40 that is contacted byan engagement surface 42 of the stylus member head portion 30. As shownin FIG. 3, an unloaded position of the stylus member 14 relative to thehousing 12 is preferably characterized by substantially no axialclearance and no axial (compressive) loading of the head portion 30 andthe O-ring 36 between the cushion member 40 and the housing stop surface34, the cushion member 40 having a substantially planar front surface inits unloaded condition in the exemplary configuration of FIGS. 1-6.Suitable materials for the cushion member include low density siliconfoam, a preferred such material being commercially available as No.86235K42 EXOBLOC™ silicone foam from McMaster Carr of Los Angeles,Calif. Thus foam has a UL-94-HF-1V-0 rating, a density of from 9 to 19pounds per cubic foot, and requires only approximately 2 to 3 psi forcompression to 25 percent of a relaxed thickness. In the preferredexemplary configuration of the stylus 10, the cushion member 40 has athickness of approximately 0.125 inch and an outside diameter of 0.200inch, the cushion cavity 38 having the same dimensions.

The engagement surface 42 of the head portion 30 is also preferablyconfigured for producing, in combination with the cushion member 40, adesired force-displacement profile of the stylus member 14. Toward thatend, the engagement surface 42 has a first portion 42A that ispreferably barely in contact with the cushion member 40 when the stylusmember 14 is in its advanced, unloaded position shown in FIG. 3 (and bybroken lines in FIG. 5), and a second portion 42B for contacting thecushion member when the stylus member is displaced rearwardly as shownby solid lines in FIG. 5. As shown In FIG. 6, preferred configurationsof the touchpad stylus 10 provide for smoothly increasing axialdisplacement of the stylus member 14 of up to approximately 0.05 inchesin response to axial loads ranging from approximately zero up toapproximately 1000 grams. Typically, preferred configurations of thetouchpad stylus 10 provide for smoothly increasing axial displacementsof the stylus member 14 of up to approximately 0.05 inches in responseto axial loads ranging from approximately 0 up to approximately 500grams, more typically ranging from approximately 0 up to approximately200 grams, and most typically ranging from approximately 0 up toapproximately 100 grams. In the illustrated exemplary configuration, thefirst portion 42A of the engagement surface is substantially planar, thesecond portion being generally frusto-conical for producing a mildlyincreasing force-displacement profile that smoothly becomes a morerapidly increasing profile as the stylus member 14 is displaced towardthe retracted position shown by the solid lines in FIG. 5. This behavioris approximated by the upper logarithmic scale in FIG. 5. Moreparticularly, and as shown in FIG. 4, the planar first portion 42A ofthe engagement surface 42 has a diameter D1, the second engagementsurface portion 42B sloping outwardly and forwardly toward a diameterD2, the diameter D2 being coincidently an overall outside diameter ofthe stylus member head portion 30.

In the preferred exemplary configuration, the head portion 30 of thestylus member 14 is formed with the diameter D1 of the first engagementsurface portion being approximately 0.050 inch, the limiting diameter D2of the second engagement surface portion being approximately 0.104 inch(the stylus stop surface 32 radially projecting approximately 0.010outwardly from an approximate diameter of 0.094 inch of the stylusmember shank portion 28. The limiting diameter D2 of the engagementsurface second portion 42B is preferably located forwardly of the firstportion 42A by a distance equal to or only slightly greater than thefull axial travel distance of the stylus member 14 between the positionsshown in FIGS. 3 and 5, as best shown in FIG. 5. Thus in this describedconfiguration the engagement surface 42 contacts progressivelyincreasing area portions of the cushion member 40 as the stylus member14 is displaced rearwardly.

It will be appreciated that the stylus member 14 can be very low in massas compared with an aggregate mass of the touchpad stylus 10. Forexample, approximating the volume of the stylus member as beingequivalent to a diameter of 0.1 inch and a length of 1 inch, thematerial having a density of 1.6 relative to water, the approximate massis only0.1**2 (in)*1.0 (in)*2.54 (cm/in)**3*1.6*π/4=0.2053 gm.This contrasts favorably with an estimated aggregate mass of at leastabout 4 gm for a complete stylus of sufficient size to be comfortablygrasped and controlled by a user thereof, the mass ratio beingapproximately 20. It is contemplated that satisfactory operation of thetouchpad stylus 10 is obtained with the mass of the stylus member beingnot greater than 0.5 gm, although the mass is preferably not greaterthan the approximately 0.2 gm of the above-described preferred example.

With further reference to FIG. 7, an alternative configuration of thestylus member head portion, designated 30′ has a cylindrical counterpartof the engagement surface second portion, designated 42B′, for producinga nearly linear force-displacement curve as approximated by the lowerlinear scale in FIG. 6. As shown in FIG. 7, the diameter D2 is onlyslightly greater than the diameter D!, the difference being due to aslight rounding of the engagement surface 42 at the outer margin of thefirst engagement surface portion 42A that is provided for preventingcutting of the pad member 40 when being deformed by the head portion 30.

As further shown in FIG. 1, the sheet member 21 that is optionallyapplied to the touchpad outer membrane 18 has a permanently appliedadhesive coating 44, the coating 44 being formulated for securelyaffixing the sheet member 21 to the outer membrane 18 of the touchpad20, yet permitting removal of the sheet member without damage to themembrane 18 and without leaving any significant portion of adhesivematerial on the membrane. A material suitable for use as the sheetmember 21 and having the adhesive coating 44 is commercially availableas WRITEWRIGHT™ from Fellowes of Itasca, Ill. This material also has aslightly textured writing surface opposite the adhesive coating 44 forfacilitating marking thereof by a suitable writing instrument.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, other shapes and sizes of the engagement surface42 are contemplated as well as different formulations and geometry ofthe cushion member as variations of the configurations described abovebeing within the ordinary skill in the art and providing, minimum stylusmember travel distances of 0.025 inch or greater with the maximum axialbias not exceeding approximately 200 grams. Also, a coil compressionspring or other type of spring member can be substituted for the cushionmember 40, and the stylus member 18 can be provided with a rolling-ballat the tip extremity 16. Therefore, the spirit and scope of the appendedclaims should not necessarily be limited to the description of thepreferred versions contained herein.

1. A stylus device for use with a touchpad device, comprising: (a) anelongate stylus housing having a longitudinal axis; (b) a stylus bodyhaving a tip portion for contacting the touchpad device, the body beingsupported by the housing for axial movement along the longitudinal axis;(c) a stop for limiting forward movement of the stylus body relative tothe housing; and (d) biasing means for forwardly biasing the body with aforce being limited to a predetermined maximum axial bias over apredetermined minimum travel distance of rearward movement relative to arest position of the stylus body, the minimum travel distance being notless than 0.05 inch, the maximum axial bias being not more thanapproximately 1000 gm.
 2. The stylus device of claim 1, wherein thestylus body has a mass being less than 10 percent of an aggregate massof the stylus device.
 3. The stylus device of claim 2, wherein the massof the stylus body is not greater than 0.5 gm.
 4. The stylus device ofclaim 2, wherein the mass of the stylus body is not greater thanapproximately 0.2 gm.
 5. The stylus device of claim 1, wherein thestylus body has a mass being less than approximately 5 percent of anaggregate mass of the stylus device.
 6. The stylus device of claim 1,wherein the biasing means comprises a resilient cushion memberinterposed between a forwardly facing cavity surface of the housing anda rear extremity of the stylus body.
 7. The stylus device of claim 6,wherein the rear extremity the stylus member forms an engagement surfacehaving a progressively increasing cross-sectional engagement area overan axial distance not less than the minimum travel distance, the cushionmember having a contact area facing the stylus member, the contact areabeing greater than a maximum of the engagement area.
 8. The stylusdevice of claim 7, wherein the engagement surface comprises a faceportion and a tapered portion, the face portion being generally planarand oriented approximately perpendicular to the longitudinal axis. 9.The stylus device of claim 7, wherein the contact area of the cushionmember is at least approximately 4 times the maximum of the engagementarea.
 10. The stylus device of claim 9 wherein the cushion member has anaxial depth being not less than double the minimum travel distance. 11.The stylus device of claim 9, wherein the maximum of the engagement areais not more than approximately 0.01 square inch.
 12. The stylus deviceof claim 6, wherein the minimum travel distance is at leastapproximately 0.05 inch and the maximum axial bias is not more thanapproximately 500 gm.
 13. The stylus device of claim 1, wherein the stopcomprises a resilient stop member interposed between a housing stopsurface of the housing and a stylus stop surface of the stylus body. 14.The stylus device of claim 13, wherein the stylus body has a cylindricalstem portion, the stylus stop surface forming a generally annularenlargement of the stylus body, the housing stop surface being generallyannular and facing the stylus stop surface, the stop member beingprotruded by the stylus body.
 15. The stylus device of claim 1, whereinthe stylus body comprises a polymeric material having a frictionalcoefficient not greater than 0.2.
 16. The stylus device of claim 1,wherein the tip portion of the stylus body forms a contact surfacehaving a radius of curvature being not less than 0.5 mm.
 17. The stylusdevice of claim 16, wherein the radius of curvature of the contactsurface is at least 1.0 mm.
 18. The stylus device of claim 1, incombination with the touchpad device, the touchpad device having anactive surface and being response to force locally applied against theactive surface, and further comprising a flexible sheet materialremovably covering the active surface to provide a replaceable writingsurface to be contacted by the tip portion of the stylus.
 19. A stylusdevice for use with a touchpad device, comprising: (a) an elongatestylus housing having a longitudinal axis and a forwardly facing cushioncavity, and a rearwardly facing annular housing stop surface; (b) astylus member supported by the housing for axial movement along thelongitudinal axis, the stylus member being formed of a polymericmaterial having a frictional coefficient not greater than 0.2, andcomprising: (i) a cylindrical stem portion; (ii) a tip portion having acontact surface adapted for contacting the touchpad device, the contactsurface having a radius of curvature not less than 0.5 mm; (iii) aforwardly facing generally annular stylus stop surface; and (iv) anengagement surface extending forwardly from a rear extremity the stylusmember, the engagement surface having a cross-sectional engagement areaprogressively increasing to a maximum engagement area of not more thanapproximately 0.01 square inch over an axial distance of not less than aminimum travel distance of the stylus member; (c) a resilient annularstop member interposed between the housing and stylus member stopsurfaces for limiting forward movement of the stylus member relative tothe body; and (d) a resilient foam polymer cushion member in the cushioncavity and having a cushion surface contacting the engagement surface ofthe stylus member, the cushion surface having an area not less thanapproximately 4 times the maximum engagement area of the stylus memberfor forwardly biasing the body with a force being limited to not morethan approximately 500 gm over the minimum travel distance of the stylusbody, the minimum travel distance being not less than approximately 0.05inch.
 20. The stylus device of claim 19, wherein the engagement surfacecomprises a face portion and a tapered portion, the face portion beinggenerally planar and oriented approximately perpendicular to thelongitudinal axis, the face portion having an area of approximately 25percent of the maximum engagement area.